This invention relates generally to integrated circuit Zener diodes and more particularly to integrated circuit Zener diodes having subsurface or "buried" breakdown junctions.
As is known in the art, one type of semiconductor voltage reference device is a so-called Zener or avalanche diode. In one type of such diode, a p-type conductivity base region is diffused through a surface of an n-type conductivity epitaxial layer which itself is formed on a p-type conductivity silicon wafer. Subsequently, an n-type conductivity emitter region is then diffused within the base region through the same surface. A p-n junction is thus formed between the base-emitter regions, and when a proper voltage is applied across the junction, the junction breaks down to establish the reference or Zener voltage. Unfortunately, this junction breakdown occurs along the surface of the silicon and, therefore, the precise breakdown or reference voltage is susceptible to surface effects, thereby affecting the accuracy of the reference voltage. Further, in such a surface breakdown device, the breakdown or reference voltage is generally relatively noisy and does not have relatively long-term stability, thereby limiting its effectiveness. Still further, because the surface is especially sensitive to contamination in an oxide layer generally formed on the surface, or the silicon-silicon dioxide interface, short term instabilities and "turn on" drift may result in the reference voltage, thereby adversely affecting the usefulness of such a device in many applications.
Subsurface junction breakdown devices have been described to reduce these effects. One such device has been suggested in a paper entitled "Monolithic-Temperature Stabilized Voltage Reference With 0.5 ppm/.degree.C. Drift" by Robert C. Dobkins, 1966 IEEE International Solid State Circuits Conference, pgs. 108-109. As discussed therein, a deep P+ type conductivity region is diffused into an N-type conductivity silicon wafer, such diffusion then being covered with a standard base diffusion followed by an N+ type conductivity emitter diffusion which covers the deep P+ type conductivity diffusion. Another such diffusion type device is described in U.S. Pat. No. 4,136,349 issued Jan. 23, 1979, inventor Wei K. Tsang, entitled "IC Chip With Buried Zener Diode". In both such vertical diffusion vertical structure type devices, the breakdown occurs in the region where the doping concentration is greatest, that is, across the junction between the P+ and N+ type conductivity regions; hence, beneath the surface of the silicon. However, because of the symmetrical nature of these vertical devices, the Zener breakdown does not occur at one point but rather can occur at or "hunt to" various points along the junction resulting in time related voltage changes and long term noise (i.e. low frequency noise). One Zener diode having its p-type region configured as a "finger" that extends from the larger p-type region into the n-type region to increase the current density through the "finger" and thereby reduce noise is described in connection with FIG. 4 in an article entitled "A Five-Terminal .+-.15-V Monolithic Voltage Regulator" by William F. Davis, published in the IEEE Journal of Solid State Circuits, Vol. SC-6, No. 6, December 1971, pgs. 366-376. However, such described diode is not a "buried" device and hence is subject to the aforementioned surface effects.
Another type of buried Zener diode includes an implantation layer disposed beneath the surface of the structure and passing laterally through laterally spaced apart, surface diffused, p and n type conductivity regions. Such devices are described in an article entitled "An Ion-Implanted Subsurface Monolithic Zener Diode" by Sik Lui, Robert G. Meyer and Norman Kwan, published in the IEEE Journal of Solid-State Circuits, Vol. SC-14, No. 4, August 1979, pgs. 782-784; and, U.S. Pat. No. 4,079,402 entitled "Zener Diode Incorporating an Ion Implanted Layer Establishing the Breakdown Point Below the Surface" inventors James L. Dunkley and James E. Solomon, issued Mar. 14, 1978. In such implanted buried Zener diodes while the breakdown point for avalanching occurs below the surface and hence away from surface contaminants such as are found in the oxide surface layer, because of the configuration of the p and n type conductivity regions, the actual point of avalanche can vary with time as in the case of the diffusion formed buried zener devices described above.
As is also known in the art, one type of zener diode is a so-called Kelvin zener diode. Such a device has a pair of anodes; one called the force anode or electrode and the other the sense anode or electrode. Such a device is referred to in U.S. Pat. No. 4,313,083 entitled "Temperature Compensated IC Voltage Reference", inventors Barrie Gilbert and Peter R. Holloway, issued Jan. 26, 1982 and U.S. Pat. No. 4,315,209 entitled "Temperature Compensated Voltage Reference Circuit" by James C. Schmoock, issued Feb. 9, 1982 and assigned to the same assignee as the present invention. As is known, the force electrode is used to apply a sufficient voltage potential relative to the cathode potential to break down the Zener junction while the sense electrode is used merely to sense the breakdown voltage.